The present invention relates to a method for blow molding hollow plastic articles, and more particularly to improvements in a method and apparatus for blow molding plastic articles which constitute or resemble bottles, canisters or analogous containers wherein a neck portion defines a passage for introduction or evacuation of flowable contents.
In many presently known blow molding apparatus, a mold having partible mold sections is employed to transport sections of a continuously or intermittently extruded tubular parison from an extruding station to a blowing station at which a blowing and calibrating mandrel penetrates into one end of the confined parison section to thereby shape the neck portion of the article. The making of the article is completed by blowing air into the interior of the parison section so that the latter conforms to the outline of the cavity in the mold. The mold is thereupon opened and the finished article removed from the mandrel so that it can descend onto a conveyor or into a stationary receptacle. The parison section which is transported by the mold from the extruder toward a position of register with the mandrel may be open at one or both ends and may be partially expanded to form a so-called bubble or cell prior to removal from the extruding station.
The mandrel which is caused to penetrate into the open end of the parison section or bubble in the mold cavity has a calibrating portion which causes the material of the parison section to move radially outwardly and to completely fill the space between such calibrating portion and the adjacent sections of the so-called neck mold which may be integral with or is separably secured to the corresponding sections of the blow mold. This insures that the neck portion of the thus-deformed parison section or bubble assumes a predetermined configuration, e.g., a tubular formation having external threads which can mesh with the internal threads of a cap or the like. Complete filling of the just-mentioned space with plastic material is desirable and necessary in order to insure that each article of a series of successively produced articles is formed with a neck portion of identical shape. It will be seen that, in contrast to the formation of the major part of a blow molded article (by expansion in response to introduction of a gaseous blowing medium into the parison section below the neck mold sections), the neck portion of the article is formed by press molding, i.e., by mechanically expanding the plastic material of that part of a parison section in the cavity of the blow mold which receives the calibratiing portion of the mandrel. Reference may be had to commonly owned U.S. Pat. No. 3,209,401 to Mehnert.
The just-described press molding of neck portions of blow molded articles invariably produces some surplus which extends upwardly beyond the neck portion and is normally separated from the neck portion by an annular cutting edge or shoulder of the mandrel in cooperation with a complementary surface or shoulder of the neck mold. As a rule, separation of the usually annular surplus is incomplete, i.e., the surplus adheres to the upper edge face of the neck portion by one or more thin webs. This is due to the fact that the severing action of the cutting edge is actually a squeezing or pinching action, not a true severing action. It is already known to retain the annular surplus on the mandrel until after the finished article is removed from the cavity of the blow mold and is caused to move out of the path of the surplus so that the latter can be separated from the mandrel and allowed to descent by gravity. This is disclosed, for example, in German Offenlegungsschrift No. 2,060,586.
Additional surplus develops when the articles are formed by blow molding of parisons whose outer diameter exceeds the inner diameter of the neck portion of the article, i.e., when the diameter of the parison must be reduced in the region which is to form the neck portion of the final product. The just-described mode of operation results in the formation of so-called "ears" which are flat pieces of surplus material extending substantially radially of the neck portion and filling specially designed recesses or pockets between the neck mold sections. In many instances, the ears extend beyond the neck mold section toward the main portion of the mold cavity so that the just-mentioned pockets must register with similar pockets between the main sections of the blow mold. Reference may be had to German Offenlegungsschrift No. 1,479,744.
The presently known proposals for removal of annular and flat surplus material which develops in the region of neck portions of blow molded articles are not entirely satisfactory for a number of reasons. Some of these proposals can be carried out by resorting to very complex and expensive auxiliary equipment which is prone to malfunction and requires frequent maintenance, especially when the surplus material is to be separated with a certain delay following expulsion of finished articles from the cavity of the blow mold in order to insure that the surplus material cannot adhere to the material of finished articles, for example, because the articles and/or the pieces of surplus material are still in a deformable state and tend to stick to each other.